There are things about driving that you can learn from a text book, but then there are certain things you can only truly learn from driving on the road. What is that one rule of the road that every driver needs to know?

There is one thing that I see daily. It's in the text books. It's a law. Yet barely anyone adheres to it. And it's when people drive in the left lane for no reason. It actually makes me visibly angry. The left lane is for passing, the right and middle lanes are for driving. By just putting along in the left lane, you disrupt traffic. You might not understand it from reading it in a book and you might not even get it from being on the road.